1. Field
The present invention relates to a ferromagnetic amorphous alloy ribbon for use in transformer cores, rotational machines, electrical chokes, magnetic sensors and pulse power devices and a method of fabrication of the ribbon.
2. Description of Related Art
Iron-based amorphous alloy ribbon exhibits excellent soft magnetic properties including low magnetic loss under AC excitation, finding its application in energy efficient magnetic devices such as transformers, motors, generators, energy management devices including pulse power generators and magnetic sensors. In these devices, ferromagnetic materials with high saturation inductions and high thermal stability are preferred. Furthermore, the ease of the materials' manufacturability and their raw material costs are important factors in large scale industrial use. Amorphous Fe—B—Si based alloys meet these requirements. However, the saturation inductions of these amorphous alloys are lower than those of crystalline silicon steels conventionally used in devices such as transformers, resulting in somewhat larger sizes of the amorphous alloy-based devices. Thus efforts have been made to develop amorphous ferromagnetic alloys with higher saturation inductions. One approach is to increase the iron content in the Fe-based amorphous alloys. However, this is not straightforward as the alloys' thermal stability degrades as the Fe content increases. To mitigate this problem, elements such as Sn, S, C and P have been added. For example, U.S. Pat. No. 5,456,770 (the '770 patent) teaches amorphous Fe—Si—B—C—Sn alloys in which the addition of Sn increases alloys' formability and their saturation inductions. In U.S. Pat. No. 6,416,879 (the '879 patent), the addition of P in an amorphous Fe—Si—B—C—P system is taught to increase saturation inductions with increased Fe content. However, the addition of such elements as Sn, S and C in the Fe—Si—B-based amorphous alloys reduces the ductility of the cast ribbon rendering it difficult to fabricate a wide ribbon. Also, the addition of P in the Fe—Si—B—C-based alloys as taught in '879 patent results in loss of long-term thermal stability which in turn leads to increase of magnetic core loss by several tens of percentage within several years. Thus the amorphous alloys taught in the '770 and '879 patents have not been practically fabricated by casting from their molten states.
In addition to a high saturation induction needed in magnetic devices such as transformers, inductors and the like, a high B—H squareness ratio and low coercivity, Hc, are desirable with B and H being magnetic induction and exciting magnetic field, respectively. The reason for this is that such magnetic materials have high degree of magnetic softness, meaning ease of magnetization. This leads to low magnetic losses in the magnetic devices using these materials. Realizing these factors, the present inventors found that these required magnetic properties in addition to high ribbon-ductility were achieved by maintaining the C precipitation layer on ribbon surface at a certain thickness by selecting the ratio of Si:C at certain levels in an amorphous Fe—Si—B—C system as described in U.S. Pat. No. 7,425,239. Furthermore, in Japanese Kokai Patent No. 2009052064, a high saturation induction amorphous alloy ribbon is provided, which shows improved thermal stability of up to 150 years at 150° C. device operation by controlling the C precipitation layer height with addition of Cr and Mn into the alloy system. However, the fabricated ribbon exhibited a number of surface defects such as scratches, face lines and split lines formed along the ribbon's length direction and on the ribbon surface facing the casting atmosphere-side which is opposite to the ribbon surface contacting the casting chill body surface. Examples of a split line and face lines are shown in FIG. 1. The basic arrangement of casting nozzle, chill body surface on a rotating wheel and resulting cast ribbon is illustrated in U.S. Pat. No. 4,142,571.
Thus, there is a need for a ferromagnetic amorphous alloy ribbon which exhibits a high saturation induction, a low magnetic core loss, a high B—H squareness ratio, high mechanical ductility, high long-term thermal stability, and reduced ribbon surface defects with high level of ribbon fabricability, which is the primary aspect of the present invention. More specifically, a thorough study of the cast ribbon surface quality during casting led to the following findings: the surface defects started in early stage of casting, and when the defect length along ribbon's length direction exceeded about 200 mm or defect depth exceeding about 40% of the ribbon thickness, the ribbon broke at the defect site, resulting in abrupt termination of casting. Because of this ribbon breakage, the rate of cast termination within 30 minutes after cast start-up amounted to about 20%. On the other hand, for the ribbon having saturation inductions of less than 1.6 T, the rate of cast termination within 30 minutes was about 3%. In addition, on these ribbons, defect length was less than 200 mm and defect depth was less than 40% of the ribbon thickness with defect incidence being one or two at every 1.5 m along ribbon's length direction. Thus, reduction of surface defects formed along ribbon's length direction in a ribbon with saturation inductions exceeding 1.6 T is clearly needed to achieve continuous casting, which is yet another aspect of the present invention.